This invention relates to the rotating pressure screening of screening paper fiber stock.
The term "foil" or "impulse member" as used herein is not necessarily limited to hydrodynamically-shaped elements or elements that resemble air foil sections, although the latter are commonly used for impulse foils in rotating apparatus for screening paper fiber stock. However, these terms are intended to apply to any kind, and any shape, of a disrupter which may be, for example, a protuberance on the surface of a drum-type rotor.
In the pressure screening of paper fiber stock for removing impurities or for fractionating suspensions of paper fibers in a pulp slurry, it is common practice to provide a rotor that has multiple impulse members mounted on arms or mounted on a support drum, with respect to a discreet area or surface of a screen. Thus, as an example in such a pressure screen, equally spaced arms and attached foils may be mounted for rotation in close proximity to a screen surface, as shown for example in Weber U.S. Pat. No. 4,166,028, Martin U.S. Pat. No. 4,851,111 or Chupka U.S. Pat. No. 5,078,275. In such apparatus, as few as two arms and foils in diametric relation have been used, and arrangements with five, eight and eleven foils, have been tried. In those embodiments in which the foils are supported on radially extending arms, the use of three or more such arm-supported foils has become a preferred and common configuration. This preference has been used, in part, to create a more uniform distribution of impulse forces at the screen itself, so as to reduce screen flexure, stress, and possible breakage due to metal fatigue. On drum style rotors, two or more foil elements are used, for any given axial position or region of the rotor or cylinder.
Cylindrical screen baskets of the kind represented in U.S. Pat. Nos. 4,166,028 and 4,851,111 are commonly fed with incoming stock suspension at one axial end of the screen, as at the upper end, or at the lower end, as shown in the examples of these patents. The stock consistency is at its lowest value at the inlet end of a screen, and by reason water extraction through the screen the consistency increases to a maximum at the outlet end. The screening energy that is imparted to the stock by the rotating foils must be sufficient as to disrupt the fiber network and break up fiber flocs to allow the individual fibers and fines to flow through the openings or slots of the screen. Also, this energy causes contaminants within the fiber matrix to loosen and be separated by the screen from the accepted fiber.
Since screening is energy costly, it is desirable to have the energy intensity in the boundary layer on the screen plate surface close to the minimum that is required for the necessary fluidization of the fiber mass. If the energy intensity is too great, mixing turbulence occurs which reduces effective screening. On the other hand, if the energy intensity imparted by the foils is too low in the boundary layer at the screen inlet surface, the screening process becomes ineffective, and primarily only washing and dewatering of the fiber suspension will occur.
Since conventional screen rotors create a uniform impulse intensity over the axial length of a cylindrical screen, greater energy intensity is imparted to stock at the inlet end or inlet zone than is actually necessary to fluidize the lower consistency pulp. As a result, energy is wasted, and contaminants may be pushed through the screen openings. When an attempt is made to balance the condition by controlling the energy input, then the energy intensity at the outlet end can become too low to properly fluidize the thickened stock. Accordingly, a more energy effective screening can be accomplished by tailoring the rotor and foil design to individual screening zones.
Compensation for the change in stock consistency along the axial length of a cylindrical screen has been attempted by varying the spacing of protuberances on a drum-type rotor to a maximum clearance at the input end and a minimum clearance at the output end. However, this compensation technique is relatively ineffective since it results in a substantial spacing of the impulse inducing members from the screened surface at the inlet end, and well beyond the boundary layer at the screen inlet surface where fiber mat disruption is required for effective screening.
Accordingly there is a need for improved rotor designs which provide efficient screening with less overall energy.